CURING KINETICS AND THERMAL-PROPERTIES OF VINYL-ESTER RESINS

The curing kinetics of dimethacrylate-based vinyl ester resins were st udied by scanning and isothermal DSC, gel time studies, and by DMTA. T he rate of polymerization was raised by increased methyl ethyl ketone peroxide (MEKP) concentration but the cocatalyst, cobalt octoate, reta rded the reaction rate, except at very low concentrations. By contrast , the gel time was reduced for all increases in either peroxide or cob alt concentration. This contradictory behavior was explained by a kine tic scheme in which the cobalt species play a dual role of catalyzing the formation of radicals from MEKP and of destroying the primary and polymeric radicals. The scanning DSC curves exhibited multiple peaks a s observed by other workers, but in the present work, these peaks were attributed to the individual influence of temperature on each of fund amental reaction steps in the free radical polymerization. Physical ag ing appeared to occur during the isothermal polymerization of samples cured below the ''fully cured'' glass transition temperature (T-g). Fo r these undercured materials, the difference between the DSC T-g and t he isothermal curing temperature was approximately 11 degrees C. Dynam ic mechanical analysis of a partially cured sample exhibited anomalous behavior caused by the reinitiation of cure of the sample during the DMTA experiment. For partially cured resins, the DSC T-g increased mon otonically with the degree of cure, and this dependence was fitted to an equation related to the Couchman and DiBenedetto equations. (C) 199 7 John Wiley & Sons, Inc.